Q&A: What does my BMI really mean?

Q: I just used an online BMI calculator and it said I was overweight! Should I be worried?

A: The BMI–or Body Mass Index– is one of many measurements that clinicians use to evaluate your overall risk for chronic diseases like diabetes, hypertension and heart disease. It is a calculation that takes into account the ratio of your weight to your height, which at the population level has been shown to correlate to disease risk.

A BMI over 25 is considered to be ‘overweight’–meaning that there is shown to be an elevated risk of disease in groups of people at that BMI level.

A BMI over 30 is considered to be obese, and suggests another cutoff point at which an even higher risk of disease is observed.

Interestingly, health risk also increases at very low BMIs: a BMI less than 18.5 is considered underweight and is undesirable as well.

BMIs between 18.5 and 24.9 are considered to be “normal,” meaning that there is no increased risk of health issues associated with people in that category.

However, at the individual level, BMI alone is not predictive of whether a certain person is at increased risk for disease, because there are a lot of factors that influence the calculation itself–as well as factors that influence disease beyond weight–which make BMI an imperfect indicator of an individual’s health risk on its own.

For example:

BMI does not account for body composition, only total body weight. If you’re an athlete or weightlifter carrying around a lot of muscle, your BMI may suggest you’re overweight, but given your high percentage of lean mass and low percentage of body fat, you would not be considered at increased risk for disease based on your BMI.

Weight is just one factor that contributes to disease risk; cardiovascular fitness level counts for a lot, too. Research has shown that fit people who are considered obese based on their BMIs still have significantly lower risk of disease and death than unfit people with “normal” BMIs. In other words, improving one’s cardiovascular fitness level through exercise at all BMI levels has real and important health benefits, regardless of whether the exercise produces weight loss.

For these reasons, BMI is but one of several metrics we use to determine someone’s individual risk for disease, and should only be considered in the context of factors such as race, fitness level, waist circumference, family history of disease, smoking status and clinical test results like cholesterol levels and blood pressure. If you’re concerned about whether your individual BMI places you at risk for disease down the road, make an appointment with your doctor to discuss whether weight loss would benefit you; and rest assured that if you are overweight, even a modest weight loss of 5-10% of total body weight is associated with significant improvements in health risk and chronic disease symptoms.